1. Technical Field
The present invention relates to an automotive disk brake, and in particular, to a disk brake with an electric parking brake having an electric parking brake function.
2. Prior Art
Generally, an automotive disk brake comprises: a pair of pads disposed on opposite sides of a disk; and a caliper in which a piston is slidably provided in a cylinder having a closed end, such that a hydraulic pressure, when introduced into the cylinder, thrusts the piston to cause the pair of pads to press against the disk, thereby exerting a braking force. Recently, however, disk brakes with an electric parking brake, that is, disk brakes further equipped with an electric parking brake function, have come into use. An example of a conventional disk brake having an electric parking brake is disclosed in Domestic Announcement (KOHYO) No. HEI 05-506196 of PCT application. In this example, an electric motor, which is provided outside the cylinder, functions as a driving source, such that the piston, which has been thrust by a hydraulic pressure introduced into the cylinder, is mechanically held in a braking position even after the hydraulic pressure is no longer applied.
Commonly, a disk brake with an electric parking brake is incorporated into an automatic brake system such as a vehicle dynamics control system, an antilock brake system (ABS), or a traction control system. In such a case, a hydraulic pump in the automatic brake system is used as a hydraulic pressure source for introducing a hydraulic pressure into a cylinder of the disk brakes (see Domestic Announcement No. HEI 05-506196 of PCT application). Normally, an automatic brake system of this type is provided with disk brakes with an electric parking brake at rear wheels and general-purpose disk brakes at front wheels.
However, the hydraulic pump used in the above-mentioned automatic brake system is not capable of generating a very large hydraulic pressure, which is required when a parking break is in operation, and thus is generally not suitable for use as a hydraulic pressure source for a disk brake with an electric parking brake. In a case that disk brakes with an electric parking brake are incorporated into an automatic brake system, one measure is to enlarge a diameter of piston, whereby the piston can be subject to increased thrust via a pressure receiving area of the piston.
However, to optimize a distribution of braking forces between front brakes and rear brakes in a brake system, it is common for a diameter of pistons of disk brakes with an electric parking brake disposed at rear wheels to be made smaller than that of pistons of general-purpose disk brakes disposed at front wheels. Therefore, in a case that a piston diameter is increased as described above, an optimal distribution of braking forces between the front brakes and rear brakes cannot be attained when normal, so-called service brakes are in operation, thus having an adverse effect on the entire brake system.
The present invention has been made in view of the above-described problem associated with conventional brake systems. An object of the present invention is to provide a disk brake with an electric parking brake that is capable of exerting a large piston thrust required for operating parking brakes, without adversely affecting the service brakes, so as to ensure a stable balance between the service brakes and the parking brakes.